Improve the test case from 5668a857d

Improve the test case from 5668a857d

In commit 5668a857d, we fixed an issue with incorrect results in right
semi joins and introduced a test case to verify the fix.  The test
case involves SubPlans and InitPlans, which may not be immediately
apparent in relation to the issue we addressed.

This patch simplifies the test case with a more straightforward query.

Per discussion with Melanie Plageman.

Author: Richard Guo
Discussion: https://postgr.es/m/CAAKRu_a-Cip2XCXp13fmxq+T9BhLAVApHTyjr94awL2mbXHC-Q@mail.gmail.com